The present invention relates to a parabolic antenna for circular polarized wave of a satellite broadcast system in SHF band (3 GHz-30 GHz) and, in particular, relates to a primary feeder for such an antenna. The present primary feeder relates in particular to a backfire helical antenna.
A conventional circular polarized wave antenna for SHF band has been disclosed in the Japanese patent laid open publication No. 93402/81, in which an endfire helical antenna is used as a primary feeder as shown in FIG. 1, in which the numeral 1 is a parabolic reflector. An endfire helical antenna 2 is located at the focal point of the reflector 1. The endfire helical antenna 2 is coupled with a coaxial cable 3 which functions as a feeder line.
However, an endfire helical antenna as shown in FIG. 1 has the disadvantage in structure, as a coaxial cable 3 must traverse a reflector surface, because the antenna 2 is fed at the far end from the reflector surface. Therefore, the coaxial cable 3 prevents the reflected wave path or blocks the wave, thus, that feeder line deteriorates the characteristics of the antenna itself. Further, the length of the coaxial cable 3 in that structure must be long, and the long feeder line increases the power loss of transmission signal. Further, the mechanical strength for supporting the endfire helical antenna together with the feeder line poses some problems to be solved in the prior art.